Extraction method

ABSTRACT

In extracting a solute from an aqueous solution using a mixer-settler extractor and an organic extractant comprising a mixture of a tertiary amine and an organic diluent, the extractor is operated while the liquid phase inside the mixer thereof is kept in a state of solvent dispersion. Thus, the mixer-settler extractor can be operated efficiently.

TECHNICAL FIELD

This invention relates to an extraction method in which a mixer-settlerextractor is efficiently operated in extracting a solute from an aqueoussolution using an organic extractant containing a tertiary amine.

BACKGROUND ART

In comparison with a tower extractor, a mixer-settler extractor isdisadvantageous in that it is generally more expensive to install andthe system has a greater quantity of hold-up. In carrying out anextraction operation with a smaller number of stages, however, theinstallation cost becomes lower, on the contrary, and the mixer-settlerextractor provides quite excellent advantages for a system in which theextraction speed is low, one in which the difference in viscosities isgreat between oil and water and one contaminated with impurities. In amethod of extracting a solute from an aqueous solution using an organicextractant consisting of a tertiary amine and an organic diluent, theviscosity of the extractant is high and, in addition the speed of thesolute transfer is low, so that a tower extractor not provided with amobile part, as represented by a perforated plate tower and a packedtower, is not generally employed. In other words, the actual height pertheoretical stage becomes extremely high in these types of extractorsand hence, an extractor having an extremely large tower height becomesnecessary. Moreover, the operating range is narrow in order to preventflooding and the column diameter of the tower becomes uneconomicallylarge.

In carrying out the extraction efficiently using an organic extractantsystem containing a tertiary amine, therefore, it is more advantageousto use an extractor which is designed to increase the dispersioncoalescence frequency of droplets, such as a mixer-settler extractor.

The problem encountered in operating a mixer-settler extractor in thissystem is liquid separation in the settler. If the stirring power isincreased so as to improve the extraction efficiency in the mixer, thesolution becomes an emulsion or reaches a state analogous to anemulsion, so that an extended period of time is necessary for the liquidseparation in the settler. This results in the necessity of using anextremely large settler, with consequent increase in the quantity ofhold-up and increase in the installation cost and in the quantity ofinitial liquid charge.

To improve the liquid separation, various methods have been employed.For example, the extraction temperature is raised or the stirringcondition in the mixer is optimized to prevent the occurrence ofundesirable fine droplets and to steepen the distribution of dropletsdiameters. Alternatively, a recycle pump is disposed between the mixerand the settler of each stage and recycles the solvent phase or thewater phase so as to keep the phase ratio (of the solvent phase to thewater phase) inside the mixer at an appropriate level. A method ofelevating the separation speed of two liquids by adding an additive suchas an alcohol, an alkali metal salt or an alkaline earth metal salt (seethe Japanese Patent Publication No. 36209/1974) and an electrical methodwhich is generally employed in a demelter of an oil refinery have alsobeen applied with certain successes. However, the effects brought forthby these methods are not entirely satisfactory in the system to whichthe prevent invention is directed.

DISCLOSURE OF INVENTION

The inventors of the present invention have carried out intensivestudies on the liquid separability in the settler in recovering a solutein an aqueous solution using a mixer-settler extractor and an organicextractant comprising a mixture of a tertiary amine and an organicdiluent and have found that the greatest factor in improving the liquidseparation speed is one other than the general, well-known onesdescribed above, that is, maintaining the organic extractant (solvent)phase as the dispersed phase inside the mixer, or operating the mixerwhile the liquid phase in the mixer is kept in a state of a solventdispersion. The present invention is based on this finding.

Thus, the present invention relates to a method of extracting a solutefrom an aqueous solution using an organic extractant comprising amixture of a tertiary amine and an organic diluent in a mixer-settlerextractor, characterized in that the extractor is operated while theorganic extractant phase inside the mixer is the dispersed phase and theaqueous solution is the continuous phase of a dispersion.

According to the observation of the inventors of the present invention,when the operation is effected with the water phase as the dispersedphase in the mixer (that is, in the case of a water dispersion), theliquid separation in the settler is effected in such a manner that waterdroplets in the organic extractant phase fall down by gravity against anupward flow of the continuous organic extractant phase. This situationis generally given by Stokes' formula or the like and the quantity ofthe water phase entrained by the organic extractant phase can bedetermined by this formula. Generally, the droplets generated inside themixer are liable to exhibit a relatively wide diameter distribution,though depending upon the stirring condition of the mixer, so that finewater droplets are unavoidably entrained by the organic extractant phaseand the liquid separation inside the mixer becomes insufficient,resulting in a poor extraction efficiency. Accordingly, it is importantto narrow the diameter distribution of the dispersed droplets inside themixer and yet to keep a large average diameter of the droplets, in orderto improve the liquid separation efficiency inside the settler. Thisalso holds true exactly of the case where the extractor is operated inthe state of a organic extractant dispersion with the solvent phasebeing present as the dispersed phase inside the mixer. The presentinventors have found that the speed of aggregation and coalescence ofthe droplets varies remarkably even under the stirring condition of themixer which provides the same droplet diameter distribution, dependingupon whether the dispersed phase is either the water phase or theorganic extractant phase.

The present inventors have carried out quantitative examination of theliquid separation behavior after stopping the stirring using a mixer andhave found that the liquid separation speed could be more drasticallyimproved by the solvent dispersion than by the water dispersion underthe condition where the stage efficiency at one mixer-settler stage iskept virtually at 100%. Selection of the dispersed systems has oftenbeen considered in the past from the aspect of mass transfer. It has notbeen known at all to this date to select the dispersed systems in viewof the liquid separability in the settler as in the present invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram schematically showing the range of dispersion insidea mixer; and

FIG. 2 is a diagram showing liquid separation speeds in an embodiment ofthis invention and in a comparative example.

In the stirring operation of heterogeneous liquid-liquid systems ingeneral, determination of the dispersed system inside the mixer isgoverned by the proportion of volumes of the liquids inside the mixer.In the case of a water phase and an organic extractant phase, this canbe divided into three ranges as schematically depicted in FIG. 1.

In FIG. 1, the ordinate represents the ratio of the volume of the waterphase to the total volume of the liquids inside the mixer and theabscissa represents the operating parameter of a mixer, such as thenumber of revolutions for stirring or the temperature. The dispersedphase in each of regions (A), (B) and (C) is as follows:

Region (A):

The quantity of the water phase is greater than that of the organicextractant. In this case, the organic extractant phase serves as thedispersed phase inside the mixer whereas the water phase can not.

Region (C):

The quantity of the water phase is smaller than that of the organicextractant phase. In this case, the water phase serves as the dispersedphase inside the mixer whereas the organic extractant phase can not.

Region (B):

The relation of the quantities between the water phase and the organicextractant phase is in between cases (A) and (C). In this case, thewater phase can serve as either the dispersed phase or the continuousphase. The organic extractant phase can also serve as the dispersedphase or the continuous phase.

Generally, the liquid inside the mixer of a mixer-settler extractor ismostly operated in region (B), so that the organic extractant phase canserve as the dispersed phase or the water phase can serve as thedispersed phase. The boundary between regions (A) and (B) can bedetermined by the steps of charging first the organic extractant intothe mixer, adding gradually the aqueous solution at a constantrevolution speed and a predetermined temperature where complete mixingis attained, stopping the addition and stirring of the aqueous solutionwhen the water phase is replaced by the organic extractant phase servingas the dispersed phase, and measuring the proportion of the water phaseinside the mixer. The boundary between regions (B) and (C) can bedetermined by the steps of charging first the aqueous solution into themixer, then adding gradually the organic extractant under the samecondition as described above and measuring the proportion of the organicextractant phase when the organic extractant phase is replaced by thewater phase serving as the dispersion phase.

As can be understood from the above description, a solvent dispersioncan be attained in the system of the present invention by charging inadvance the aqueous solution into the mixer and then adding the mixedorganic extractant consisting of a tertiary amine and an organic diluentwith stirring. Alternatively, the mixer is half filled with water andthen the aqueous solution and the organic extractant are chargedtherein. In his case, it is advisable to charge initially water into thesettler. The operation is carried out so that the proportion ofquantities of the water phase and the organic extractant phase insidethe mixer falls within the region (B) in FIG. 1.

The method of the present invention is useful for extracting andrecovering an acid such as acetic acid, acrylic acid, or nitric acidfrom an aqueous acidic solution using an organic extractant consistingof a tertiary amine and an organic diluent. The present method iseffective when applied to an aqueous acid solution having an acidconcentration of up to 50 wt %.

The method of the present invention is particularly effective forrecovering acetic acid from an aqueous acetic acid solution using anorganic extractant consisting of a tertiary amine and an organicdiluent. In other words, acetic acid is extracted from the aqueoussolution using a mixer-settler extractor and a tertiary amine such astri-n-octyl amine (TOA) in combination with an organic diluentcomprising an oxygen-containing organic solvent such as3,3,5-trimethylcyclohexanone (TMCH) as the organic extractant (wherebythe amine and the solvent have boiling points higher than that of aceticacid).

The tertiary amines to be used are preferably those having a boilingpoint higher than that of acetic acid and form a non-aqueous phase. Thenumber of carbon atoms contained in the tertiary amine should be around12 to 40, considering the low solubility in the water phase and theseparability from acetic acid by distillation. It is also preferable inorder to obtain a large apparent distribution coefficient that thetertiary amine has no larger branches near the nitrogen atom. It isundesirable to have an ethyl or larger substituent group on a carbonatom adjacent to the nitrogen atom or spaced from the nitrogen atom byone CH₂ group.

It is also to be avoided to have a benzyl group or those having a cyclicstructure near the nitrogen atom. In other words, the tertiary aminesshould be selected, from those which have a partial structurerepresented by >N--CH₂ --CR¹ R², wherein R¹ is a hydrogen atom and R² isa hydrogen atom or a methyl group, for example, C₆ or highertrialkylamines such as trihexylamine, trioctylamine, triisooctylamine(tris-2,4,4-trimethylpentylamine), trilaurylamine, dimethyllaurylamine,dimethylhexadecylamine, methyldi(tridecyl)amine, or dimethyldecylamine;tertiary amines having an alkenyl group such as dimethyloleylamine orbutylbis(5,5,7,7-tetramethyloct-2-en-1-yl)amine (XE-204); or tertiaryamine mixtures such as dimethylcocoamine, dimethyl(C₈₋₁₂ alkyl)amines,or dimethyl(hydrogenated tallow)amine. Commercially available tertiaryamines can be used as such. It is also possible to obtain tertiaryamines by alkylating primary or secondary amines available asintermediates by a known method. Among many usable tertiary aminesmentioned above, TOA is readily available and shows an excellentapparent distribution coefficient when it is admixed with anoxygen-containing organic solvent to form an extractant.

Examples of the organic solvent to be used in combination with theamines are those oxygen-containing organic solvents which have a boilingpoint higher than acetic acid, e.g. ketones, alcohols, carboxylicesters, or phosphoric esters. When these solvents are used incombination with the tertiary amines described above, acetic acid in theaqueous solution can be extracted with a particularly large apparentdistribution coefficient.

EXAMPLE 1

150 cc of a 1 wt % aqueous acetic acid solution was charged in advanceinto a mixer consisting of a separable glass flask having an innerdiameter of 75 mm and a height of 75 mm and pitched paddles (four)having an agitation blade length of 40 mm and a width of 8 mm. While theaqueous solution was being stirred at 700 r.p.m., 150 cc of a mixedorganic extractant (50:50 volume ratio) consisting of a tertiary amine(TOA) and an organic diluent (TMCH) was added portionwise to allow themixed organic extractant to serve as the dispersed phase. In this case,confirmation was made with naked eye that the organic extractant servedas the dispersed phase. After stirring and mixing were effected at thesame revolution speed for 10 minutes, stirring was stopped and therelation between the ratio of volumes of the clear water phase and theorganic extractant phase to the total volume and the time was measured.Here, the temperature was controlled to 30° C. The result is shown bysolvent dispersion line in FIG. 2. As can be seen from this diagram,complete liquid separation could be attained after about 10 seconds.

COMPARATIVE EXAMPLE 1

150 cc of the mixed organic extractant consisting of the same tertiaryamine and the same organic diluent as those used in Example 1 wascharged in advance into the same apparatus as used in Example 1. Whilethe mixture was being stirred at 700 r.p.m., 150 cc of a 1 wt % aqueousacetic acid solution was added portionwise to permit the 1 wt % aqueousacetic acid solution to serve as the dispersed phase. After stirring andmixing were effected for the same period of time, stirring was stoppedto carry out the same measurement. The temperature was also controlledto the same temperature as above. The result is shown by the aqueousphase dispersion line in FIG. 2. As can be seen from this diagram, about110 seconds of time was necessary for attaining complete liquidseparation and the separation speed was remarkably lower than that ofthe solvent dispersion.

What is claimed is:
 1. A method of recovering a solute from an aqueoussolution thereof, comprising the steps of: feeding into a mixer of amixer-settler extraction system(1) said aqueous solution, and (2) aliquid organic extracting agent capable of extracting said solute fromsaid aqueous solution, and mixing said aqueous solution and saidextracting agent in said mixer so as to form a dispersion in which thedisperse phase is droplets of said extracting agent which droplets aredispersed in a continuous phase of said aqueous solution, whereby thesolute is extracted from said aqueous solution and is transferred tosaid extracting agent, the volumetric ratio of the aqueous solution (1)to the total liquids in said mixer being at a level effective tomaintain said dispersion in a stable condition in said mixer; then, inthe settler of said extraction system, allowing said dispersion tosettle whereby to obtain separately (i) an aqueous phase and (ii) aliquid organic extracting agent phase containing the solute.
 2. A methodas claimed in claim 1 in which, before said extracting agent is fed intosaid mixer, said mixer is partially filled with said aqueous solutionand then the remainder of said aqueous solution and said liquid organicextracting agent are fed into said mixer.
 3. A method as claimed inclaim 1 in which, before said extracting agent is fed to said mixer, theentirety of said aqueous solution is placed in said mixer and then saidliquid organic extracting agent is fed into said mixer.
 4. A method asclaimed in claim 1 in which the solute is acetic acid and the liquidorganic extracting agent comprises a tertiary amine having a boilingpoint higher than the boiling point of acetic acid.
 5. A method asclaimed in claim 4 in which the acid concentration of said aqueoussolution of acetic acid is not more than 50% by weight.
 6. A method asclaimed in claim 4 in which said liquid organic extracting agentconsists essentially of said tertiary amine and an oxygen-containingorganic material having a boiling point higher than the boiling point ofacetic acid.
 7. A method as claimed in claim 4 in which said liquidorganic extracting agent consists essentially of tri-n-octylamine and3,3,5-trimethylcyclohexanone.